1. Field of the Invention
The present invention relates to technology for regenerating a printing plate, for future reuse, including a photosensitive layer which has a photocatalyst and on which an image forming material of an organic compound or the like is applied. More particularly, the present invention relates to a printing press, and an apparatus and a process for regenerating a printing plate which employs the above technology.
2. Description of the Related Art
In recent years, digitalization of printing process has become progressing in the art. This technology involves creation of images and manuscripts in digitized form on a personal computer or reading images on a scanner and directly makes a printing plate based on the digital data thus obtained. This makes it possible to save labor in the printing processes and also to conduct high-precision printing with ease.
So-called PS plates (presensitized plates) have been commonly used as printing plate to date. A PS plate includes a hydrophilic non-image area made of anodized aluminum and has one or more hydrophobic image areas formed by curing a photosensitive resin on the surface of the anodized aluminum. Making of a printing plate with such a PS plate requires plural steps and hence, is time-consuming and costly. It is, therefore, difficult to reduce the time and the cost required for printing processes. Especially in short-run printing, the requirement for numerous steps causes increased printing costs. Additionally, since use of a PS plate requires a development step using an alkaline developer, serious problems arise not only with the need for considerable amounts of labor but also with environmental pollution caused by treatment of developer waste.
Further, it is a common practice to expose a PS plate whose surface is in contact with a film, through which a desired image is perforated, to light. This causes problems in making the printing plate directly from digital data and in promoting digitized printing processes. Moreover, after completion of printing of a pattern, it is necessary to replace the printing plate with another one in order to conduct printing of the following pattern, and used printing plates are thrown away.
To solve the above-described problems of PS plates, processes have been proposed to meet the digitization of printing processes while making it possible to omit the development step, and some of such processes have come into commercial use. For example, Japanese Patent Application Laid-Open (KOKAI) Publication No. SHO 63-102936 discloses a process of making a plate which comprises the steps of: applying ink containing a photosensitive resin, as an ink for a liquid ink-jet printer, onto the surface of a printing plate; and curing an image area by irradiation with light. Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-254633, on the other hand, discloses a process for making a color offset printing plate by an ink-jet head through which solid ink is jetted.
Also included in known processes are a process for making a printing plate, which comprises the step of wirting with a laser beam an image on a printing plate, which is made of a PET (polyethylene terephthalate) film on which a laser absorbing layer such as carbon black covered with a silicone resin layer is formed, to cause the laser absorbing layer to evolve heat, which ablates off the silicone resin layer; and another process for preparing a printing plate, which comprises the step of coating a linophilic laser absorbing layer on an aluminum plate, coating a hydrophilic layer on the laser absorbing layer, and then ablating off the hydrophilic layer with a laser beam as in the above-described process.
In addition, a process has also been proposed for the making of a printing plate made of a hydrophilic polymer by exposing the hydrophilic polymer imagewise such that the hydrophilic polymer is oleophilized.
Further, a process of directly forming an image on a PS plate with light is also proposed. For this purpose, an imaging unit utilizes a blue laser having a wavelength of 402 nm, and a so-called CTP (Computer To Plate) device including a micromirror and a UV (ultraviolet) lamp have been put on the market.
But, since each of the above-mentioned processes cannot continue to print a following pattern until a printing plate is replaced by another one subsequent to completion of printing of a pattern, they are not different from the conventional art in that the printing plate is thrown away after its one-time use even if it is possible to make printing plates directly from digital data.
On the other hand, a technique including regeneration of a printing plate has been disclosed. For example, Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 10-250027 refers to a latent image block copy making use of a titanium dioxide photocatalyst, a making process of the latent image block, and a printing press on which the latent image block is made. Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-147360 also discloses an offset printing process by a printing plate making use of a photocatalyst. Each of these disclosures writes an image using light (practically, ultraviolet light) to activate a photocatalyst and regenerate a printing plate by hydrophobization of the photocatalyst caused by heat treatment. Further, Japanese Patent Application Laid-Open (KOKAI) Publication No. HEI 11-105234 discloses a making process for a lithographic printing plate, which comprises the step of hydrophilizing a photocatalyst with activating light, i.e., ultraviolet rays, and then writing an image area by heat-mode recording.
As disclosed in the paper (pages 124–125) entitled “Study on Behavior of Photoinduced Hydrophilization Associated with Structural Change in Titanium Deoxide Surface”, (by Sanbe et al.) distributed at the Fifth Symposium on “Recent Developments of Photocatalytic Reactions” of the Photo Functionalized Materials Society in 1998, Prof. Fujishima, Prof. Hashimoto, et al. of the Research Center for Advanced Science and Technology, The University of Tokyo, have confirmed hydrophilization of a titanium dioxide photocatalyst by heat treatment. According to the description in the above paper, the processes disclosed in the laid-open patent applications referred to in the above, that is, the processes each of which hydrophobizes a photocatalyst by heat treatment to regenerate a printing plate cannot regenerate to reuse a printing plate or make a printing plate.
With the foregoing problems in view, Inventors have been enthusiastically researching to develop a printing plate, on which an image is rapidly formed with an imaging unit utilizing the activating light or light (i.e., inactivating light) having a wavelength longer than that of the activating light and which is rapidly regenerated after completion of printing, and a process for making and regenerating such a printing plate.
As a result of the research, Inventors have conceived a process for regenerating a printing plate by applying an organic compound that causes hydrophobization onto the surface of the printing plate. In this process, regeneration largely depends on the speed of decomposition of the organic compound by the photocatalyst under irradiation with the activating light.
Japanese Patent Application Laid-Open (KOKAI) Publication No. 2000-6360 discloses a method to delete an image formed on a printing plate by irradiating a layer including a photocatalyst of the printing plate with activating light mainly including ultraviolet light.
Another Japanese Patent Application Laid-Open (KOKAI) Publication No. 2002-1900 states that, if a hydroplilic-and-oleophilic material applied on the surface shows photothermal effect, irradiation with heat rays such as infrared light deletes an image formed on the surface of a printing plate.
These processes however take a considerable time to delete an image formed on the printing plate.